Boreal bog plants: nitrogen sources and uptake of recently deposited nitrogen

The main goals of this study were to determine the (delta)15N signature of quantitatively important boreal bog plants as basis for discussing their N sources, and to assess the effects of five different 3 year N treatments (i.e. 0, 5, 10, 20 and 40 kg N ha-1 year-1) on the bog plants and surface peat at different depths (i.e. 0, 5, 10, 20 and 40 cm) by using 15N as tracer. Plants and peat were analyzed for N concentration, 15N natural abundance and 15N at.%. From the results we draw three main conclusions: First, the relative importance of different N sources is species-specific among bog plants. Second, an annual addition of 5 kg N ha-1 year-1 was sufficient to significantly increase the N concentration in Sphagnum mosses, liverworts and shallow rooted vascular plants, and an annual addition of 40 kg N ha-1 year-1 during 3 years was not sufficient to increase the N concentration in deep rooted plants, although the 15N content increased continuously, indicating a possible longer term effect. Third, an annual addition of 40 kg N ha-1 year-1 during 3 years increased the N content in surface peat at depths of 5 and 10 cm, but not at depths of 20 and 40 cm, indicating the capacity of the living Sphagnum mosses and the surface peat to take up deposited N, and thereby function as a filter.